1. Field of the Invention
The invention relates to a flash discharge lamp which is used, for example, for heat treatment of semiconductor substrates and liquid crystal substrates and for similar purposes. The invention relates especially to a flash discharge lamp in which the outside surface of the arc tube is provided with a trigger electrode.
2. Description of Related Art
Conventionally, a flash discharge lamp is common in which the outside of the arc tube in which a pair of opposed electrodes is arranged is provided with a trigger electrode.
Furthermore, a lamp is known in which, within a sealed tubular body of silica glass, a trigger electrode is sealed and in which this sealed tubular body is located along the arc tube of the flash discharge lamp (hereinafter also called “lamp”).
This technology is described in Japanese Patent Application JP-A-2003-203606 and corresponding U.S. Pat. No. 6,960,883.
A conventional flash discharge lamp is described below using FIG. 7. FIG. 8 is an enlarged cross section for describing the hermetically sealed arrangement of the sealed tubular body as shown in FIG. 7. In this flash discharge lamp, within the tubular arc tube 2 of silica glass, there is a pair of electrodes 1. On the outside of the arc tube 2 of this lamp, there is a trigger electrode 3 which is a metallic tungsten rod.
The trigger electrode 3 is located within a sealed tubular body 4 formed of a cylindrical silica glass tube the ends of which are sealed. One end 31 of the trigger electrode 3 is connected to a metal foil 33, a lead 34 which projects from the sealed tubular body 4 is connected to its other end. By hermetic pinch sealing of the sealed tubular body 4 in the region of the metal foil 33, the trigger electrode 3 is held sealed within the sealed tubular body 4. The inside of the sealed tubular body 4 is filled with inert gas and is subjected to a vacuum atmosphere. Thus, oxidation of the trigger electrode 3 is prevented.
The sealed tubular body 4 and the arc tube 2 are attached to one another by a nickel attachment component 5. The attachment component is not shown in FIG. 8.
One end 31 of the trigger electrode 3 is attached to the sealed tubular body 4 by hermetic pinch sealing of the sealed tubular body 4. The other end 32 of the trigger electrode 3 is the free end within the sealed tubular body 4. In this arrangement, even when the trigger electrode 3 expands by receiving light from the lamp, the amount of this expansion can be absorbed by the gap between the other end 32 and the inner wall of the sealed tubular body 4.
By this arrangement in which the trigger electrode 3 is held sealed within the sealed tubular body 4, oxidation of the trigger electrode 3 or deposition of the material comprising the trigger electrode 3 on the arc tube 2 in the case of sputtering of the trigger electrode 3 at a high temperature can be prevented. As a result, formation of cracks in the arc tube 2 can also be prevented.
However, it is required of this flash discharge lamp that a semiconductor substrate (as the article to be treated) is irradiated with light with greater than or equal to 20 J/cm2 energy within the short time of 1 msec. To achieve this, the peak energy with which the flash discharge lamp is supplied is up to 5×106 W.
Therefore, since the light emitted from the lamp has high energy, the trigger electrode 3 instantaneously reaches a high temperature, expands and afterwards contracts. This means that the trigger electrode 3 often repeats expansion and contraction according to the lamp emission.
As shown in FIG. 8, in the hermetically sealed part of the sealed tubular body 4, as a result of the different coefficients of expansion between the silica glass comprising the sealed tubular body 4 and the tungsten comprising the trigger electrode 3, a very small gap is formed in the vicinity of the trigger electrode 3. Furthermore, as shown in FIG. 8 using the broken line, a region A in which the trigger electrode 3 is welded to the metal foil 33 is repeatedly exposed to tension which forms during expansion and contraction.
Furthermore, when light is emitted from the lamp in the space in the vicinity of the lamp, shock waves are formed. The effect of these shock waves causes the lamp to vibrate, together with this, also the sealed tubular body 4 and the trigger electrode 3 vibrate.
Also, since the trigger electrode 3 and the metal foil 33 are interconnected by resistance heating, the region A to which the metal foil 33 is welded is brittle. That is, in the part A in which the metal foil 33 is welded, the strength of the metal foil is less than the actual strength of the metal foil, if the expansion-contraction stress on the trigger electrode 3 and the effect of the shock waves are repeatedly applied. As a result, the metal foil 33 is shifted into the state (with a separated part) in which it can be in part easily torn.
In this state, if a high frequency high voltage is applied to the trigger electrode 3, in the separated region of the metal foil 33, a discharge is formed by which there is a case in which the trigger output decreases, and as a result, there is no lamp emission. This means that there is a case in which lamp emission takes place, and a case in which there is no lamp emission. Thus, there is the disadvantage that the operating property of the lamp becomes extremely unstable.
Furthermore, for repeated discharges in the torn part of the metal foil 33, finally, the metal foil 33 is completely torn, by which the lamp can no longer be operated at all.